Field of the Invention
The present invention relates to an illumination unit and an image display device illumination incorporating the same.
Description of the Related Art
An image display device such as a front-projection type projector is widely used for business presentation, formal education, and household use, for example, to project an image on a screen placed ahead of the device.
The brightness of an image on the projection plane of the image display device is determined by the power of a light source and optical use efficiency of an optical system. Herein, the optical use efficiency refers to a ratio of the intensity of a reflected beam to that of illumination. To improve the optical use efficiency of the image display device, it is necessary to enhance the transmittance and reflectivity of optical elements of the optical system or reduce a loss of light amount due to vignetting on the optical path.
The transmittance and reflectivity of an optical element can be improved to several % or less by applying non-reflective coating onto at least either incidence surface or exit surface thereof.
A loss of light amount due to vignetting is caused by a light-projected area larger than the effective area of the optical element. There are two kinds of such optical loss in an image display device comprising an optical tunnel and a DMD (Digital Micromirror Device), that is, an optical tunnel coupling loss and a DMD loss.
The optical tunnel coupling loss refers to a loss of light amount occurring on an optical path from light emission from a light source to light incidence on the optical tunnel.
The DMD loss is a loss of light amount occurring when a light beam reflected by the optical element of an illumination system is projected on the DMD. That is, the DMD loss occurs because a light beam projected to the DMD (hereinafter, DMD projection beam) is larger in size than the effective area of the DMD and distorted in shape relative to the effective area. Herein, the effective area refers to an area on the DMD by which the DMD projection beam can be reflected.
In view of this, it is needed to realize an illumination unit which is able to illuminate a projection plane brightly while reducing the DMD loss.
For instance, Japanese Laid-open Patent Application Publication No. 2004-184626 (Reference 1) discloses an image display device which comprises an imaging system to form an image of light from a light source and a projection system to project a formed image on a screen. It teaches an illumination system comprising a rod integrator, a relay lens, a first mirror, a second mirror, and a condenser lens.
Reference 1 also discloses that the second mirror includes a rotationally asymmetric reflective surface with a larger curvature radius corresponding to a long side of an exit of the rod integrator than a curvature radius corresponding to a short side thereof, for the purpose of acquiring a high imaging performance with a reduction of astigmatism.
Further, Japanese Laid-open Patent Application Publication No. 2002-268010 (Reference 2) teaches an illumination system comprising a cylinder mirror and a spherical mirror as the first and second mirrors, respectively.
However, Reference 1 and 2 are silent about the provision of a mirror with an anamorphic surface which can suppress the DMD loss and brightly illuminate the projection plane by optimizing the shape of the DMD projection beam.
Further, in Reference 1 the reflective surfaces of the first and second mirrors are both anamorphic and free-form surfaces with high-order extended aspheric coefficients. Therefore, it is a problem that it takes a lot of time to inspect the shapes of the reflective surfaces to determine whether they are properly formed.